In medical diagnosis, it is very important to evaluate radiographs such as X-ray images. Bones, implants or similar structures generally stand out clearly from the surrounding soft tissue, and are therefore easily perceptible. On the other hand, soft tissue structures such as tendons or blood vessels are generally reproduced only very unclearly on radiographs. In many pathologies, however, it is in fact the perceptibility of the soft tissue structures which is important. Furthermore, it is often difficult to distinguish similar types of tissue from one another. Smaller bones which are imaged over a larger bone on a radiograph, for example, can often scarcely be made out with the naked eye; the same applies for soft tissue structures. In such cases, therefore, doctors can often make no diagnosis or only a very unreliable diagnosis on the basis of the radiographs.
The digitising of radiographs has provided some degree of improvement. Using known methods of image processing, such as contrast enhancement within selected image sections, soft tissue structures can for example sometimes be emphasised clearly. In general, however, a tendon lying over a bone cannot thereby be made perceptible. This is because the smaller fluctuations of the signal level of image signal components which represent the tendon do not stand out significantly from the high background signal level of the bone. Although a monitor used for the display will in the best case still reproduce the small fluctuations of the signal level as intensity fluctuations, these are usually so small that they are scarcely perceptible to the naked eye.
The present invention is directed to resolving these and other matters.
An object of the present invention is to provide a method and a device for improving the perceptibility of different types of structures on radiographs.
This object can be achieved by a method for improving the perceptibility of different structures on radiographs by using an image processing device, comprising the following steps: storing a radiograph provided in electronic form as a position-space intensity distribution; carrying out a Fourier transformation in order to determine a frequency-space intensity distribution; filtering the frequency-space intensity distribution by modifying the weighting between high-frequency and low-frequency image signal components, the image signal components to be weighted more strongly being set by taking into account an average structure size of the structures whose perceptibility is intended to be improved; and, carrying out an inverse Fourier transformation of the filtered frequency-space intensity distribution, so as to obtain a modified position-space intensity distribution in which these structures are more easily perceptible; and/or by an image processing device for improving the perceptibility of different structures on radiographs, wherein the image processing device comprises a memory (MEM) for storing a radiograph provided in electronic form as a position-space intensity distribution; a Fourier transformation unit (FT) for carrying out a Fourier transformation in order to determine a frequency-space intensity distribution; a filter (FIL) for filtering the frequency-space intensity distribution by modifying the weighting between high-frequency and low-frequency image signal components, the image signal components to be weighted more strongly being set by taking into account an average structure size of the structures whose perceptibility is intended to be improved; and, an inverse Fourier transformation unit (FT−1) for carrying out an inverse Fourier transformation of the filtered frequency-space intensity distribution, so as to obtain a modified position-space intensity distribution in which these structures are more easily perceptible.
The invention is based on the discovery that in most cases, the structures whose perceptibility is intended to be improved differ more or less significantly in respect of their size and fineness from the other structures imaged on the radiograph. Since smaller and finer structures are manifested by higher frequencies in the Fourier spectrum than large coarse structures are, by modifying the weighting between high-frequency and low-frequency image signal components in the Fourier spectrum it is possible to enhance the image contrast either for small fine structures or for large coarse structures. Depending on whether the poorly perceptible structures are finer or coarser than the easily perceptible structures, the weighting of the image signal components in the frequency space will be modified in favour of either the high-frequency or the low-frequency image signal components.
The structures, which are poorly perceptible at first, are made to stand out clearly in particular when the image signal components to be weighted are set in that the period lengths corresponding to these frequencies are approximately twice as great as the average structure size of the structures whose perceptibility is intended to be improved.
In one particularly simple filtering embodiment of the present invention, e.g., in which the filtered frequency-space intensity distribution F′(fx, fy) is given by: F′(fx, fy)=TF(fx, fy)·F(fx, fy)—where F(fx, fy) is the frequency-space intensity distribution for the position-space intensity distribution, fx, fy are frequencies in the two-dimensional frequency space and TF(fx, fy) is a filter function for the weighting of image signal components—the frequency-space intensity distribution is merely multiplied by a filter function.
By using central frequency values and profile functions for setting the frequency ranges to be weighted, the filtering can be expediently controlled with relatively few parameters in which the filter function is set by at least one central frequency value and at least one profile function, which modifies the weighting of the image signal components as a function of the distance from the central frequency value; or in which the filter function is set by at least one central frequency value and at least one profile function, which modifies the weighting of the image signal components as a function of the distance from the central frequency value.
In another aspect of the present invention, a Gaussian function is particularly suitable as a profile function, since it has the property of remaining a Gaussian function even after the inverse Fourier transformation. The filtering can then be represented in the position space as a convolution of the intensity distribution with a Gaussian function. This prevents the filtering from leading to divergence of positions in the image where the intensity distribution changes abruptly, and which therefore have a particularly high contrast.
The frequencies or frequency ranges, which have their weighting modified, are determined according to the average structure size of the structures whose perceptibility is intended to be improved. The average structure size or corresponding frequency ranges may either be fixed in advance or, freely selectable with the aid of control elements on the image processing device or via a user interface of a superordinate computer. By modifying the crucial filter parameters, a treating doctor can therefore expediently improve the perceptibility of structures in which they are interested on a very wide variety of radiographs.
Furthermore, automatic determination of the frequency ranges via an adaptive method is also feasible wherein after an operator has selected a structure whose perceptibility is intended to be improved on the radiograph, the at least one central frequency value and the at least one profile function are set adaptively so that the contrast of this structure is increased.
The structures whose perceptibility is intended to be improved may, for example, be selected by specifying a point on the boundary of the structure and a direction, along which the contrast is intended to be increased; or by specifying two points within the structure, between which the contrast is intended to be increased.
In a further embodiment of the present invention, additional high-frequency filtering in which the frequency-space intensity distribution F(fx, fy) is also subjected to high-frequency filtering—for example in which the high-frequency filter is given by a Gaussian filter with the central frequency value 0 leads to an increase in the signal-to-noise ratio since image signal components reflecting image structures become enhanced relative to high-frequency background noise. Such filtering compensates for the fact that the Fourier amplitudes decrease with an increasing frequency f in the images often to be represented in practice.
The advantageous configurations and advantages mentioned above in respect of the method also apply accordingly for the image processing device according to the invention.
Other features and advantages of the invention will be found in the following description of an exemplary embodiment with the aid of the drawing, in which: